The present invention relates to a resistor composition for resistor-incorporated spark plugs and, more specifically, to improvements in the resistance properties of the resistor composition upon sealing.
Generally, conventional resistor compositions for resistor-incorporated spark plugs have been basically made up of a mixture of inorganic components (hereinafter referred to as the base mixture), viz., glass frit and inorganic aggregate, the said aggregate substantially comprising alumina, zircon, mullite, silica, fused silica, clay, magnesia, silicon mitride, boron nitride, aluminium nitride or the like or a mixture of two or more of these substances, and a carbonaceous material including carbon black, acetylene black, graphite or other organic substance capable of being carbonized by calcination or sealing by heating such as, for instance, glycerol, methyl cellulose, polyvinyl alcohol and the like. To further improve the service life under discharge load of the resistor, one may use per 100 parts by weight of the aforesaid base mixture, 0 to 30 parts by weight of one or more components selected from the group consisting of the oxides and carbides of rare earth elements and metals (Ti, Nb, Cr, etc.) from Subgroups IVa, Va and VIa of the long period type periodic table (Iwanami Rikagaku Jiten, third edition, pp. 1484-5) and the carbides of B or Si.
As illustrated in the accompanying single drawing, a center bore 2 of a refractory insulator 1 forming part of a spark plug is charged with such resistor compositions, which are usually sealed between a center electrode 3 and a terminal rod 7 by means of conductive sealing glass powders 4 and 6 to form a resistor body 5.
This resistor body has to be sealed so as to attain a given resistance value with a view to preventing the occurrence of radio frequency interference on ignition of the spark plug. However, conventional resistor compositions have varying resistance values depending upon the magnitude of sealing temperature, and render it difficult to obtain a desired constant resistance value, thus requiring stringent sealing temperature control for their preparation.